Did you consider grouping instead of split pushing?
IT is based off of how well you play that champion compared to other players. I had an 8/6/11 urgot game where I did nothing special and got an S-, this last game was 9/8/17 as vlad S+
You need a good KDA compared to other Sorakas. Wards placed/killed (including pinks), Objectives killed/assisted to kill, CS.
This is all mainly compared to people who play the champ you play in the role that you play them. Going 1/0/15 on Soraka is an A grade because "better" Soraka players might go 2/0/16 or 5/6/18 with help with 4 dragons, baron, and 5 towers, 30+ wards placed, 10+ wards cleared and hit S rating because it's all a comparison.
Your Soraka rating is just A compared to others. I'm going to be guesstimating for this next part. The top 2% receive S+ Top 5% are S Top 12-18% are A+/- Top 19-40% are B+/- Top 41-60% are C+/- The rest are D+/-
I honestly think from experience that if you get a kill on a support, you won't get an S.